To return a thing to a known starting state. A reset discards what accumulated, on purpose.
Re- (Latin, again) attached to set, from Old English settan (to place, to set), from Proto-Germanic satjan; to reset is to place a thing back at a known starting point, discarding whatever had accumulated since.
To return a thing to a known starting state. The root's placing is done on purpose here — a reset discards what accumulated, because starting clean was the point.